1. Field of the Invention
The present invention relates to implantable medical devices. More specifically, the present invention relates to battery-powered implantable devices that receive energy for recharging the battery from an external source. Still more particularly, the invention relates to systems for transmitting energy from an external source to an implanted medical device and for transmitting data between an implanted device and external apparatus.
2. Description of the Relevant Art
Implantable medical devices, such as pacemakers and defibrillators, typically are powered by a battery that is incorporated within the device. Non-rechargeable batteries are commonly used to power the implanted devices. Non-rechargeable batteries, however, have a limited life and thus surgery, including its associated risks, discomfort and cost, is necessary to replace an implanted device once its battery is drained. Because of limited life and other undesirable consequences of using non-rechargeable batteries, the use of rechargeable batteries is desirable. Whereas operational life of an implanted device incorporating a non-rechargeable battery was limited to the duration of the original battery charge, an implanted device using a rechargeable battery can function for significantly longer periods given that the batteries can be recharged repeatedly.
One technique for recharging an implanted devices battery involves transcutaneous energy transmission, a technique which allows non-invasive battery charging. Using transcutaneous energy transmission, such as described in U.S. Pat. No. 5,411,537, an alternating current (AC) in an external primary coil of wire creates a magnetic field which, in turn, induces an AC electrical current in a secondary coil of wire that is housed within the implanted medical device. Charging energy is thus transmitted in the same manner as between the primary and secondary coils of a transformer. The alternating current induced in the implanted secondary coil is then rectified and regulated to provide direct current (DC) power for charging the medical device's battery.
Transcutaneous energy transmission, although generally safe and reliable, is not without certain shortcomings. For example, the efficiency of transcutaneously inducing a current in the implanted coil is detrimentally effected if the internal and external coils are not properly aligned or oriented, or if the distance between the external and internal coils is too great. Because there is no direct physical connection between the external charger and the implanted device to provide feedback, ascertaining whether transmission efficiency is maximized or whether the battery has become fully charged is problematic.
Also, as mentioned previously, transcutaneous energy transmission relies upon a magnetic field to induce an AC current in the implanted coil. At the same time, the alternating magnetic flux generated by the AC current may induce eddy currents in the medical device's metal housing and in the metal casings of various components internal to the implantable device. The magnitude of these eddy currents is a function of the frequency and magnitude of the magnetic flux. Eddy currents cause a temperature increase in the metal components in which the current is conducted. If too great, the temperature increase in the implanted device caused by eddy currents can damage the surrounding body tissues. A high charging current, moreover, creates large temperature rises, thereby increasing the risk of harm to surrounding tissues.
Another known recharging technique uses direct electrical connections between an external power source and an implanted receptacle. For example, U.S. Pat. No. 4,941,472 (Moden, et al) describes an implanted electrical access port to provide a receptacle for receiving needle electrodes. The electrical access port in Moden is electrically interconnected to an implanted medical device. L-shaped needle electrodes of Moden are inserted through the patient's skin and body tissue and inserted into opposite ends of the access port. A center conduit in the needle electrode is made of a conducting material and, except for the needle's tip, is surrounded by an insulating material. The Moden needle electrodes mate in the access port with brush-shaped contact assemblies. Because of the shape of the needle electrode in Moden (L-shaped), insertion of the needle electrodes is cumbersome. Further, as best shown in FIGS. 4 and 5 of Moden, the needles must be inserted into the access port completely and with a small angular tolerance. That is, it is easily possible to insert Moden's needle electrodes into the access port at such an angle that the electrode's tip will not mate with the brush-shaped, contact assembly. In this event, the required electrical connection would not be made. Also, Moden contemplates positioning the access port apart from the implanted medical device, thus requiring two surgical sites in order to implant the entire system.
U.S. Pat. No. 5,205,286 (Soukup, et al.) discloses a subcutaneous data port that provides a plurality of conductive ports for receiving needle electrodes. Multiple needle sticks are required with the Soukup device in order to mate the needles with all of the conductive ports, thus potentially increasing discomfort to the patient. Soukup also contemplates implanting the port separately from the implanted therapeutic device such that incisions in at least two locations are required.
Thus, there remains a need in the art for a system that overcomes these and other problems associated with existing systems for providing recharging current to implanted devices. A means for providing direct electrical connection between the external charging device and the implanted device would eliminate alignment concerns, eliminate the potential for tissue damage caused by the eddy currents generated by transcutaneous energy transmission, eliminate the need for inclusion of internal charging circuitry within the implanted device, and would provide a direct connection between the external charger and battery so as to provide feedback information on the status of battery charging. It would be desirable to provide a system for making a direct electrical connection between an external charging device and an implanted device which minimizes the number of surgical sites required. In particular, it would be desirable to construct an implantable medical device (a pacemaker or defibrillator, for example), that itself includes at least one receptacle, for receiving needle electrodes for recharging a battery in the medical device. It would also be desirable to minimize the number of needle electrodes required to make the required electrical connections, yet at the same time allow for multiple conductors to connect to the implantable device. It would be preferable if all the electrical connections could be made by means of a single needle. It would be further advantageous to provide direct electrical connections to an implantable medical device for, not only recharging the batteries in the medical device, but also other electrical functions such as transferring data to and from the medical device. Despite the substantial advantages that would be afforded by such a system, to date no such system has been developed.